Anionic polymerization techniques have been used to synthesize polymers that are useful in the manufacture of tires. Using these techniques, certain organometallic compounds can be used to initiate the polymerization of monomer such as conjugated diene monomer. Due to the mechanism by which the initiation and polymerization proceeds, the organometallic compound adds to monomer to form a polymer chain wherein the organo substituent of the initiator is attached as the head group of the polymer. Common initiators include organo lithium species such as n-butyl lithium.
Certain initiators impart a functional group to the polymer. These functional groups may include a heteroatom or metal that can have a desirable impact on the polymer or compositions containing the polymer. For example, where the polymers are employed in the manufacture of tire treads, the functional group can lower the hysteresis loss of the tread vulcanizate. This lowering of hysteresis loss may result from interaction between the functional group and the filler, although other mechanisms have also been proposed.
Tributyl tin lithium compounds have been used to initiate conjugated dienes (optionally together with copolymerizable monomer) to form vulcanizable polymers (i.e. rubber) that, when used in treads, have a desirable impact on the performance of the tread. Likewise, lithiated cyclic imines (e.g. lithio hexamethyleneimine) have also been used to initiate the polymerization of similar polymers and provide rubber with desirable performance in tire treads. Still other examples include lithiated thioacetals (e.g. 2-lithio-1,3-dithianes). Still further, the use of lithium dialkylphosphines in conjunction with phosphine oxide modifiers have been proposed.
The selection of useful initiator compounds, however, is not trivial. This is especially true where there is a desire to select initiator compounds that have a desirable impact on filled rubber compositions or vulcanizates, such as tire treads. Indeed, the prior art only includes a few types of compounds that are useful. This difficulty derives from several factors. For example, the anionic polymerization of conjugated dienes is sensitive, and many compounds or substituents can poison the polymerization system. And, the selection of substituents or functional groups that can impact filled compositions, such as tire treads, is difficult to predict.
Because functional initiators remain desirable, particularly for the synthesis for functionalized polymers that are used in the manufacture of tires, there is a continued desire to identify initiators that can lead to technologically useful polymers and that have desirable impact on filled rubber compositions and/or vulcanizates.